The heel is generally the first part of the foot to impact the ground. This heel strike, as it is generally known, places extra stress on the heel and can lead to unnecessary repetitive motion injury. Shoe designers have long recognized that shock absorption and diffusion are necessary to reduce the stress on initial impact of the heel. Means have therefore been provided within the soles of shoes for some of the impact on the heel to be absorbed by the sole and diffused away from the heel area. The attempt has been to spread the force concentrated in a small area of the heel over a larger area to avoid a local "sore spot".
Past attempts to diffuse the force of a heel strike have usually involved the addition of complicated structures to the shoe which added additional and sometimes complicated manufacturing steps. For example, in U.S. Pat. No. 1,566,364, Blair discloses a spring member made of steel which is disposed between the outsole and inner lining and extends into a recess under the heel. The spring member in conjunction with optional sponge rubber inserted in the recess is said to relieve the shock of the heel strike. As another example, in U.S. Pat. No. 5,185,943, Tong et al. disclose providing cushioning, stability and support in an athletic shoe by adding an insert which must be positioned between the midsole and outsole or encapsulated within the midsole or outsole. It is also known in the prior art to add a steel shank between the insole and outsole to support the shank area of the shoe and the arch of the foot. It is further known in the prior art to incorporate a tuck support into the insole board such as with a reinforced piece of texon board.
Basic shoe construction techniques have to some degree added inherent shock diffusion properties to the shoes manufactured by them. For example, a basic cement construction technique involves tacking an insole board to the shoemaker's last and cementing the marginal edges of the shoe upper--so called "lasting margins"--to the insole board. Such shoes are completed by the addition of an outsole and optionally a midsole. In addition, a sock liner or full orthotic was added to complete the interior of the shoe. Special purpose footwear incorporated lining materials to insulate the shoe and/or add waterproof-breatheability characteristics to the upper.
The use of a traditional full length insole board is a relatively poor structure to serve as the foundation for a shock diffusion system, even if combined with an appropriate sock liner or orthotic and a midsole with the appropriate elastic/damping properties. The material commonly used for such prior art insole boards (e.g. Texon board) is not sufficiently rigid to properly transmit and spread the localized force to the entire midsole because it must flex in the forefoot region in order to accommodate a normal walking or running gait. In addition, the use of a full length insole board does not permit the designer to select the area of the midsole which will bear the primary load in diffusing the shock of the initial heel strike.
The conventional use of a flat last to assemble shoes has also hindered the ability to diffuse the force of heel strike because the flat lasting technique does not permit the use of an insole board contoured to the shape of the foot. As a result, the force of the heel strike remains concentrated in the vicinity of the local sore spot.
There is therefore a need for achieving increased shock diffusion of the initial heel strike but limiting the shock diffusion to the heel and arch areas of the foot without compromising the maximum flexibility required in the forepart region of the shoe.
Another problem that has been addressed by the footwear industry is excess pronation (i.e., the outward rotation or twisting of the foot) which causes injuries. Solving this problem has led shoe designers to incorporate complex pronation control devices. For example, in U.S. Pat. No. 4,288,929, Norton et al. disclose a device which comprises an additional plastic piece attached between a lasted upper and the outsole, the plastic piece having medial and lateral walls formed around the heel. Another pronation control device, which is incorporated into the midsole, is disclosed in Kilgore et al. in U.S. Pat. No. 5,247,742.
It is therefore also advantageous to incorporate a device for reducing excessive pronation without adding additional components.